Process of recovering maleic acid from the fume gases containing small amounts thereof



Nov. 2,1937., A. o. JAEGER |:1 Al. 2,098,047 PROCESS OF RECOVERINGMALEIC ACID FROM THE FUME GASES CONTAINING SMALL AMOUNTS THEREOF l FiledNov.- 21, 1935 BY A v 7 Maf/cf .4c/ Pfam/fir 20 l @Mm l l 25W ATTORNEY.

CLI

Patented Nov. .2, 19537 .UNITED STATES PATENT vorf-"ics PROCESS OFRECOVERING MALEIC ACID FROM THE FUME .GASES CONTAINING SMALL AMOUNTSTHEREOF Application November 21, 1935, Serial No. 50,839

11 Claims. (Cl. 26o-T122) This invention relates Ato the recovery ofmaleic acid from the exhaust gases produced in the vapor phase catalyticoxidation of. naphthalene to phthalic anhydride and similar vapor phaseoxidations of organic compounds containing conjugated double bonds.

When naphthalene or similar aromatic comf with the nature of thecatalyst and which can also be increased by the addition of certaincompounds which oxidize directly to maleic acid, such as benzene,phenol, cresols, furane bodies, and the like. In the catalytic oxidationof benzene and phenols,` maleic acid is produced directly as the primaryproduct. 'I'he vapor from a converterl in a vapor phase catalyticoxidation plant is normally led through condensers -where the majorportion of the primary product is condensed as a solid. Maleic anhydridebeing relatively volatile passes through the condensers which aremaintained ata regulated, sufficiently high temperature to condenserelatively pure primary product and has to be washed out of the gases asit otherwise produces a severe fume nuisance. Even where benzene,phenols or similar compound arer directly oxidized to maleic anhydridetherel is also a considerable loss in the exhaust gas due to thevolatility of the anhydride.

In the past, ,the fume nuisancev which arose from escape of maleicanhydride into the atmosphere necessitated the scrubbing' of the gasesin so-called fume towers. The scrubbing is by water-spray or spray ofdilute alkali. In'peither case, a dilute solution of maleic acid vor itssalts was produced and allowed to run to waste because it cost more torecover the maleic'acid from the dilute fume tower liquor than it doesto produce to obtain comparable strengths bythe old watermaleic aciddirectly. In view of the great corrosiveness of maleic acid (it is oneof the strongest organic acids and is comparable to` hydrochloric acidin its corrosiveness) the discharge of the fume tower liquor presentedal considerable nuisance.- i v The present invention permits therecovery .of

maleic acid by producing a concentrated liquor y tively concentratedmaleic acid liquor will effectively scrub exhaust gases from a condensersuch as a phthalic anhydride condenser. It had been thought that ywaterhad to be used or even an alkali in order to effect sufliciently rapidsolutionv of the maleic acid from the vapors so that the fume -nuisancecould be abated and that the use of a relatively concentrated liquor asa spray would not be suitable. We have found, on the contrary, thatconcentrated spray liquor which can be circulated, angl of any strength,can be used provided a sumcient amount of spray is employed and providedthat sufficient time of exposure to the gases is obtained. c

Any suitable gas and liquid contact apparatus may be used in the placeof fume towers. Thus, for example, packed towers, self circulating gas.contactera roll spray chambers, Feld gas washers, and the like-may beemployed. In some of these devices there is a continuous circulationV ofliquor. In others, the liquor is agitated by the passage of the gastherethrough, and there is no definitely dened circulation path. Themain essential is a suicient time of contact between the maleic acidliquor and the vapors to provide for satisfactory solution offmaleicacid. When fume towers are used, the invention is preierably carried outby recirculating the liquor from the tower through spray nozzles orother distributors in the tower until a suiiicient concentration ofmaleic acid has been built up, for example, from 25 to 50%depending onthe nature of the gases, temperature, etc., and then continuously orintermittently removing a portion of the"liquor from the circuit,`replacing itwith water, dilute maleic acid liquor to be concentrated, orthe like. By this recirculation'a given volume of liquid is brought intocontact with the gases lmany times and is thereforegiven suillcient timeto .collect the necessary m`a1eic acid since it is called upon eachrecirculation only to dissolve a relatively small amount of maleic acidwhereas spray method it would be necessary to build a tower of suchenormous height'as to be economically impossible. l

In the modication using lfume towers, the present invention isparticularly concerned with the direct recirculation of the liquorleaving the lfume tower, but in more specific aspects it includescompound recirculation with ltration. This is of special value where theexhaust gases contain considerable quantities of solids which form asludge or contain such large amounts of phthalic acid as to create aproblem oi' precipiflculties are encountered, part or all of the liquorflowing fromy the tower may be subjected to ltration or settling, orboth, removing solid impurities and part or all of the filtered` liquorcan be re-introduced into the recirculation in the tower. This permits asufiiciently rapid; removal of liquid from the tower circuitvtopreventclogging through solids Without correspondingly lowering theconcentration of maleic acid in the .liquor so removed which would bethe case if the entire makeup were water.

The invention will be described in conjunction with a modern spray fumetower from a phthalic anhydride plant. The invention is, however, notlimited to this type of apparatus and any suitable gas and liquidcontact apparatus may be used. It is an advantage of the presentinvention that it is not limited to particular special apparatus,although it has been found that fume towers of the type shown in thedrawing give extraordinarily effective results with very economicalapparatus cost and in a. more specic aspect the improved apparatus isalso included in our invention.

'I'he specic description of the invention is to be taken in conjunctionwith the drawing which shows in semidiagrammatic form a fume towersystem -employing the present invention. view is a front elevation .withthe fume tower partly in section.

Exhaust vapors from a condenser from a vapor I phase phthalic anhydrideplant enter the fume tower I through the ducts 2 which are placed so asto produce a turbulent flow of vapors in the tower. 'I'he vapors ascendthe tower countercurrent to a spray of maleic acid liquor from a seriesof spray nozzles 3, suction being provided by the blower I and venturi 5on top of the tower. The liquor which scrubs the vapors removing maleicanhydride, `a portion of the phthalic anhydride and other solubleconstituents together with dust and similar insoluble solids, leaves thebottom of the tower through the pipe 6, iiowing into an overow basin orsump 1, from which it is continuously pumped by the pump I0 through thepipe I I into the manifold l2 which feeds the sprays 3. Makeup waterwhen necessary is introduced through the valved pipe 23. The excessliquor overflows from the sump 1 through the overiiow 8 into acollecting basin 9, whence it flows to a lter press I3 where solids areremoved and the ltrate passes into the pipe 24, part of it flowing intothe filtered maleic acid liquor storage tank and part being pumped bythe pump I4, through the pipe I5, into the `manifold I2. The relativeproportion flowing into the tank 20 and into the pump I 4 is determinedby adjustment of the valves 2| and 22 in the pipe 24. If desired, thewhole of the iiltered liquors may be recirculated during certainperiods. From the tank 20 the iiltered liquor may -be drawn off throughthe valved pipe to a maleic acid recovery system. .Y

In the lter press mud and crystals of phthalic acid or other materialsscrubbed 'out by the spray but less soluble than maleic acidare removedand if the proportion of phthalic acid is suiliciently high the iltercake may be treated for recovery of the values. i

Where the amount of solidsisnot suillciently great to give trouble inthe spray nozzles or where the solids are in such fine suspension asvnotto clog orifices the filter I3 may be dispensed with and the liquor fromthe collecting tank 9 permit;4

The

ted to ilow through the pipe I6 into a sedimentation tank I1, from whichit is decanted into a second sedimentation tank I8 and flows through thepipe I9 into the tank 2U.

Normally the character of the gases to be handled by the system willdetermine whether a filter press or sedimentation is to be used.Sometimes, however, it is desirable to provide both so that one systemor the other: or both together may be used. This is indicated in thedrawing by the valves 26 and 2'I in the pipe I6 and the pipe leading tothe lter press I3. In some cases where it is not desired torecoverhighly purified maleic acid, both sedimentation and lter press may bedispensed with, but normally a removal of solids in the system isdesirable.

Because of the highly corrosive nature of the maleic acid liquor,corrosion resistant construction must be used thus, for example, thefume towers may be of wood and suitable corrosion resisting pipes andspray nozzles should be employed.

The drawing illustrates the application of the present invention to asingle fume tower. Where a battery of fume towers are available theconnections may, of course, be suitably paralleled and, in general, thepresent invention is adaptable to almost any design of scrubbingequipment.

In the drawing, water is shown as added to the drawn from the systemintothe tank 20. In certain installations, however, there is availabledilute maleic acid liquor from other sources, for example, certain lowgrade portions of the solid phthalic anhydride obtained in thecondensers may be leached with a restricted amount of water to produce amaleic acid liquor or a dilute maleic acid liquor may be obtained fromsome other process in the plant such as, for example, 4the fume towersfrom sublimers for purifying phthalic anhydride where the amount ofmaleic acid present is sufficient to warrant their use and where, ofcourse, by reason of the lower concentration of maleic anhydride a moredilute liquor will be ob tained. Such liquor can, of course, replace themake up water and it is an advantage of the present invention that itcan be used to concentrate otherwise lvalueless liquor as well as toobtain from the gases a satisfactory strength of liquor in the iirstplace. In addition to producing a concentrated maleic acid liquor, thepresent inven-v tion also effects a partial purification because theaeration taking place through repeated or longf continued contact of theliquor'with air tends to coagulate certain impurities in the maleic acidliquor and transform them into such a form that they may be removed byfiltration or sedimenta- 1tion. We do not claim the aeration of maleicacid liquor per se in the present application, this con stituting thesubject matter of our copending application Ser. No. 50,838 filed Nov.21, 1935 but it is an advantage of the present invention that a partiallpuriication by aeration takes place by reason of-the very steps oi' theprocess itself. In,

obtained in the fume gases from catalytic vapor 75 phase'organicoxidation plants, which comprises scrubbing the gases with a maleic acidliquor to produce a liquor of the desired concentration.

2. A method of producing a concentrated maleic acid liquor from eilluentgases containing maleic anhydride in the concentrations normallyobtained in the fume gases from catalytic vapor phase organic oxidationplants, which comprises scrubbing the gases with maleic acid liquor,withdrawing the liquid produced, recirculating at least a portionthereof and drawing off a portion of the liquor thus produced.

3. A method of producing a concentrated maleic acid liquor from themaleic anhydride containing eiiluent gases from the condensers of avapor phase catalytic oxidation plant of aromatic compounds, whichcomprises scrubbing the vliquor from the zone of scrubbing,recirculating. at least a portion thereof yand withdrawing a por.

tion of the concentrated liquor from the circuit. 4. A method accordingto claim 3 in which the vapor phase catalytic oxidation plant is a plantin which an aromatic compound containing a major portion of naphthaleneis oxidized to phthalic anhydride.

5. A method according to claim 3 in which the vapors from the condensersare passed upwardly in countercurrent to a spray of maleic acid liquidand a suction inducing means is employed in a portion of the gas flowabove the sprays of maleic acid liquor.

maleic acid liquor, thereby concentrating the liquor, continuouslyremoving'i at least a portion thereof and withdrawing a portion of theconcentrated'liquor fromthe circuit, clarifying the withdrawn portion byseparation of solids therefrom and reintroducin'g a portion of theclarified product into the scrubbing circuit.

'1. A method according to claim s m which the vapor phase catalyticoxidation plan't is a plant in which an aromatic compound containing amaior portion of naphthalene is oxidized to phthalic anhydride.

8. A method of producing a concentrated.

maleic acid liquor from the maleic anhydride containing eiiluent gasesfrom the condensers of va vapor phase catalytic oxidation plant ofaromatic compounds, which comprises scrubbing the exhaust gases from thecondenser which contain maleic anhydride in the concentrations normallyobtained in the fume gases from catalytic vapor phase organic oxidationplants with a dilute maleic acid liquor thereby concentrating theliquor, continuously removing the concentrated liquor from the zone ofscrubbing, recir-v culating at least -a portion thereof and withdrawinga portion of the concentrated liquor from the circuit. filtering thewithdrawn portion to separate out solids and reintroducing a portion ofthe nitrate into the scrubbing circuit.

9. A method according to claim 8 in which the vapor phase catalyticoxidation plant is a. plant in which an aromatic compoundcontaining amajor portion of naphthalene is oxidized to phthalic anhydride.

10. A method of producing a yconcentrated maleic acid liquor from themaleic anhydride containing eiiluent gases from the condensers of avapor phase catalytic oxidation plant of aromatic compounds, whichcomprises scrubbing the exhaust gasesfrom the condenser which containmaleic anhydridein the concentrations normally obtained-in the fumegases from catalytic vapor phase organic oxidation plants with a dilutemaleic acid liquor, thereby concentrating the liquor, continuouslyremoving the concentrated liquor from the zonedof scrubbing,recirculating at least a portion thereof and withdrawing a portion ofthe concentrated liquor from the circuit, clarifying the withdrawnportion by subjecting to sedimentation and reintroduclng a portion ofthe clarified liquid into Ithe scrubbing circuit.

11. A method according to claim 10 in which the vapor phase catalyticoxidation plant is a plant in which an aromatic compound containing amajor portion of naphthalene is oxidized to phthalic anhydride.

ALPHoNso. JAEGER. n l

